Field of the Disclosure
The present disclosure relates generally to a method for preparing a metal oxide slurry, its use for forming an electrolyte layer having a patchwork-type surface structure with a nanoporous grain boundary, and the electrolyte layer thereby formed. The metal oxide can be gadolinium-doped cerium oxide, zinc oxide, or anything else. The metal oxide slurry prepared by the method is useful in fabricating a thin and dense electrolyte layer for fuel cells, as well as in manufacturing thin film solar cells and as a transparent conductive oxide for solar energy storage.
Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present invention.
As a renewable and sustainable energy source alternative to fossil fuel, solar energy has been attracting a lot of attention. Various technologies, including solar heating, solar photovoltaics, and solar thermal energy technologies have been utilized to capture solar energy. Although solar energy is the most abundant renewable energy, conventional solar energy devices utilize only a fraction of available solar. There is an urgent need to develop a device having high utilization efficiency of solar energy. One method to effectively capture solar energy is in the form of chemical bonds, as in photosynthesis, brought about by splitting water photoelectrochemically.
Development of semiconductors, semiconductor materials and ceramic materials capable of directly converting sunlight into fuels may provide a singular solution to convert, capture and store solar energy. Homeowners could use solar panels during the day to generate power to their home by using solar-generated stored energy to split water into hydrogen and oxygen for later combustion and further energy generation. At night, the stored hydrogen and oxgen can be recombined,using a fuel cell to generate electrical and thermal power while the solar panels would otherwise be inactive.
Transparent conductive oxide (TCO) is an optically transparent and electrically conductive doped metal oxide used for thin film silicon solar cells. TCO has to transmit as much light as possible through a substrate window to the active light-absorbing material beneath, as well as carry the current as an ohmic contact with minimal resistive losses. Indium tin oxide, fluorine-doped tin oxide, and doped zinc oxide are generally used as an inorganic film of a layer of TCO. Among them, doped zinc oxide is a strong TCO candidate to store solar energy due to its high transparency and high conductivity. Various attempts have been made to deposit a ZnO based TCO thin film to lower the resistivity.
Fabrication of thin films used as electrolytes for solid oxide fuel cells (SOFCs) has been intensively studied recently in an effort to reduce the operating temperature of SOFCs. SOFCs convert chemical energy of a fuel directly to electrical energy. Due to its high energy conversion efficiency and fuel flexibility, SOFCs have a wide variety of applications. However, SOFCs operate at a high temperature from—500 to 1000° C.—and utilize a solid oxide or a ceramic as an electrolyte. Such high operating temperature limits commercial use of this technology because acceptable performance could only be achieved from a very small number of cells, and configuration of a commercial scale power system with such a small number of cells is not cost-effective.
Ceria-based electrolytes are of current interest for application in intermediate temperature-solid oxide fuel cells (ITSOFCs) due to their high ionic conductivity. Gadolinium doped ceria (GDC), for example, has significantly higher ionic conductivity than that of yttria stabilized zirconia. Suzuki et al. have reported that microtubular cells can generate over 1 W cm−2 at 550° C. with a ceria-based electrolyte [Suzuki, T., Zahir, H., Funahashi, Y., Yamaguchi, T., Fujishiro, Y., and Awano, M., 2009, “Impact of anode microstructure on solid oxide fuel cells,” Science, 325, pp. 852-855—incorporated herein by reference]. However, at lower temperatures, the conductivity of ceria-based electrolytes significantly decreases. Ohmic losses from the electrolyte can be minimized through the use of a thinner electrolyte [see: Leah, R. T., Brandon, N. P., and Aguiar, P., 2005, “Modelling of cells, stacks and systems based around metal-supported planar IT-SOFC cells with CGO electrolytes operating at 500-600° C.,” J. Power Sources, 145, pp. 336-352; Suzuki, T, Zahir, H, Yamaguchi, T, Fujishiro, Y, Awano, M. Fabrication of micro-tubular solid oxide fuel cells with a single-grain-thick yttria stabilized zirconia electrolyte. J. Power Sources 2010; 195:7825-7828; and Suzuki, T., Zahir, H., Funahashi, Y., Yamaguchi, T., Fujishiro, Y., and Awano, M., 2008,“Fabrication and Characterization of Microtubular SOFCs with Multilayered Electrolyte,” Electro. & Solid-State Letters, 11(6), pp. B87-90; each incorporated herein by reference]. Highly dispersed nano-size GDC slurry with homogeneous distribution is indispensable for fabricating a dense and thin electrolyte layer.
Recently, in the chemical engineering and food technology fields, a wet atomizing system has been developed as a new method of mixing and dispersing [Zahir, Md. H., Suzuki, T Yamaguchi, I., Fujishiro, Y., and Awano, M., 2009 “Wet atomization of Gd-doped CeO2 electrolyte slurries for intermediate temperature microtubular SOFC application”Fuel Cells, 9, pp. 164-169—incorporated herein by reference]. Using this system, particle size reduction and homogenization are achieved within a short period of time. Therefore, attempts were made to synthesize the GDC slurries through the use of wet atomizing systems for the preparation of nanosized particles. The wet atomizing system divides the pressurized fluid in one channel and creates a cross-collision for atomization, emulsification, and dispersion. Dispersion by means of a high-pressure atomizer is performed by the large shearing force generated when a liquid is passed through an extremely narrow (small) gap at high speed. As a result, a fine homogeneous solid solution could be obtained within a very short time. It has been reported that the pore-size distributions of gamma-Al2O3 membranes with the addition of a 3.5 wt. % solution of polyvinyl alcohol (PVA) polymer do not show a measurably altered pore structure [Schoonman, J., 2003, “Nanoionics”, Solid State Ionics, 157, pp. 319-32—incorporated herein by reference]. Therefore, the optimization of the amount of binder polymer for fabrication of a smooth crack-free electrolyte layer (membrane) is important.
Zahir et al. previously reported a homogeneous GDC electrolyte slurry processing system through the use of wet-atomization; the fabricated. SOFC with the atomized electrolytes showed a maximum power density of only 350 mW cm−2 at 500° C. [Zahir, Md. H., Suzuki, T., Yamaguchi, T., Fujishiro, Y., and Awano, M., 2009 “Wet atomization of Gd-doped CeO2 electrolyte slurries for intermediate temperature microtubular SOFC application” Fuel Cells, 9, pp. 164-169—incorporated herein by reference]. However, the effect of the binder content has not yet been reported.
The present disclosure describes a method and electrolyte layer that solves the above problems. The present disclosure describes a method in which a higher amount of binder can be used to fabricate a nanoporous natural patchwork-type surface structure which has potential applications in other fields.